This invention relates to a process and apparatus for continuous casting by twin rolls that involves producing cast pieces in the form of sheets directly from molten metal.
The process for casting metal sheets by pouring molten metal into the space defined by a pair of rotating rolls opposite to each other is known as the twin roll process. In this process, molten metal is poured from above into the space defined by the two rolls positioned parallel to each other at an appropriate interval. This molten metal is brought into contact with the rolls and is cooled, with the result that a solidified shell is formed on the surface of each roll. The two solidified shells thus formed move downward as the rolls rotate and, at the same time, they increase in thickness through the heat removal by the rolls. When the two solidified shells with increased thickness reach a place where the space narrows, they are joined into one piece and rolled to a casting with a specified thickness, which is continuously withdrawn to below the rolls.
In this continuous casting process by twin rolls, molten metal also flows in the direction parallel to the roll axis when it is poured into the space defined by the rolls. For this reason, part of molten metal flows out without solidification from both ends of each roll if the rotating speed of the rolls is too low for the flow rate of molten metal.
In known casting processes, side dams are provided at both ends of each rotating roll to prevent this outflow of molten metal. Fixed side dams which are divided into top and bottom portions are used as disclosed in the Japanese Patent Unexamined Publications (called Tokkyo Kokai) Nos. 60-162558 and 61-144245, for example, and the vibrating side dam method is disclosed in Tokkyo Kokai Nos. 60-166146 and 60-170559. The process disclosed in Tokkyo Kokai No. 60-221155 is also known; in this process side dams are installed inward from both ends of each roll, not at both ends.
In the continuous caster of metal sheets disclosed in Tokkyo Kokai No. 57-94456, a pair of rotating bodies (i.e., rolls) positioned in parallel to each other are each adapted to move in the direction of axis and in the directions at right angles to this directions of axis and are used in a manner that the two rolls are mutually shifted in the direction of axis. A pair of sheet width controlling plates each of which has a circular-arc-shaped side edge conforming to the shape of the peripheral surface of the roll barrel are used. One controlling plate is positioned so as to be in contact with an end of one roll at a main surface of it and also be in contact with the peripheral surface of the other roll at the circular-arc-shaped side edge of it; the other controlling plate is positioned so as to be in contact with an end of the other roll at a main surface of it and also be in contact with the peripheral surface of the above-mentioned one roll at the circular-arc-shaped side edge of it. This construction enables metal sheets with different thicknesses and/or widths to be produced selectively.
In the continuous sheet casting by twin rolls, variable-width casting techniques are very important because requirements for widths of castings or cast products are very diverse. It is also important that castings have good edge shapes. In the casting process in which side dams are provided at roll ends, the width of a casting is equal to the roll length and it is impossible to change the width of the casting. Furthermore, when the side dam surface is shaved by fins generated between the roll end and side dam surface and by solidified metal formed on the side dam surface, molten metal flows out of a clearance resulting from this shaving or it drops onto the casting surface. As a result, it is difficult to continuously cast good sheets.
In the process in which side dams are installed inward from both roll ends, it is difficult to continuously cast sheets with good edge shapes and uniform width because fins are formed and molten metal leaks between the peripheral roll surface and the side dam edge.
When no side dams are used and the outflow of molten metal from roll ends is prevented by setting the rotating speed of the rolls at a high value relative to the flow rate of molten metal, the casting has serrated edges. In this case, therefore, it is very difficult to make uniform the casting width in the longitudinal direction of the casting and it is also exceedingly difficult to change the casting width.
When the cast piece has bad edge shapes and nonuniform widths, it is usually necessary to cut casting edges and make the width uniform in the manufacturing process, lowering the yield and adding to the number of work steps.
A casting process in which side dams are not used is also known. This process is disclosed in the specification of Japanese Patent Application (Tokugan Sho) No. 63-93060 submitted by the present applicant. This process uses a pair of rolls, each composed of a main body of stainless steel as a paramagnetic material and two ferromagnetic material (ferrite) zones that are provided along the full periphery of the main body in a manner that they are spaced each other in the direction of axis and have a relatively small width in the direction of axis. These rolls are positioned in parallel to each other with a small gap corresponding to the thickness of the cast sheet and the ferromagnetic material zones are provided opposite to each other. A pair of permanent magnet is positioned above the rolls. One of the magnetic poles of a permanent magnet faces one of the ferromagnetic material zones of one roll with a small gap therebetween, and the other magnetic pole faces one of the ferromagnetic material zones of the other roll with a small gap therebetween. One of the magnetic poles of another magnet faces the other ferromagnetic material zone of the above-mentioned one roll with a small gap therebetween and the other magnetic pole faces the other ferromagnetic material zone of the above-mentioned other roll with a small gap there between. As a result, in the positions opposite to the poles of a permanent magnet a ferromagnetic material zone of one roll is magnetized with an N pole, for example, and that of the other roll is magnetized with an S pole. This means that the mutually nearest portions of the ferromagnetic material zones opposite between the rolls are also magnetized with mutually reverse polarity. Therefore, two magnetic fields are formed between the ferromagnetic material zones spaced in two places in the direction of axis of one roll and the ferromagnetic material zones spaced in two places in the direction of axis of the other roll. These magnetic fields act on the pool of molten metal supplied from an upper nozzle toward the space defined by the rolls, enabling a cast sheet with good shapes of both edges (i.e., nonserrated edges) to be produced. The same technique as disclosed in this specification of Tokugan Sho No. 63-93060 is disclosed in "Material and Process", Vol. 1 (1988), No. 2 (Mar. 4, 1988, published by the Iron and Steel Institute of Japan, page 389) and "Casting of Near Net Shape Products" (a collection of lectures delivered at the International Symposium on Casting of Near Not Shape Products held on Nov. 13 to 17, 1988, in Honolulu, Hi., pages 583-593).
The manufacturing process of metal sheets disclosed in the Japanese Patent Examined Publication (called Tokkyo Kohkoku) No. 61-7137 (corresponding to Tokkyo Kokai No. 57-177861) is similar to these processes. Permanent magnets are used in the process disclosed in Tokkyo Kohkoku No. 61-7137. Each inside of both ends of pair of cooling drums opposite to each other is provided with a pole of a permanent magnet of mutually reverse polarity and the two poles of the permanent magnet are opposite to each other through the walls of the cooling drums. The magnetic fields formed through the walls of the cooling drums will act in the same manner as in the above-mentioned techniques. The apparatus disclosed in Tokkyo Kokai No. 63-97341 also has the same constitution as that of the technique disclosed in Tokkyo Kohkoku No. 61-7137.
Although these techniques based on the use of magnetic forces are superior to those based on the use of side dams, it is impossible to selectively produce castings with good shapes of both edges and various sheet widths.